Industrial filamentary pads used for inter-phase mass transfer contacting and/or for mist elimination are typically pads that are fabricated of multiple layers of knitted or woven metal or plastic filaments. Industrial woven or knitted-mesh pads are typically made from 4- to 11-mil diameter filaments. For fine drop removal in mist elimination, or for use as liquid-liquid coalescers, composite materials containing 10 to 50 micron diameter fiberglass or plastic filaments co-knitted with a heavier metal or plastic mesh framework are used.
In the manufacture of knitted mesh pads, the knitting machine typically knits a fabric of interlocked wire loops in the form of a continuous tube or cylinder. The mesh cylinder is utilized in a flattened form and in the case of metal wire filaments, the flattened mesh is typically crimped or corrugated to form a three-dimensional layer. Commercial filamentary pads for use in mist elimination or mass transfer are made by assembling a stack of individual layers of knitted mesh or woven filaments to form a pad of the desired depth.
The gas flow capacity limit of a filamentary pad used for mist elimination or for counter-current gas/liquid contact is set by either the flood point or re-entrainment penetration point. For the purpose of this specification, the re-entrainment penetration point is generally defined as the point at which spray generated by gas bubbling through the continuous liquid layer within the filamentary pad penetrates the upper surface of the pad.
The flood point is generally defined as the combination of gas and liquid rates at which the liquid begins to rapidly accumulate within the pad with a correspondingly rapid rise in gas pressure drop across the pad. Because entrainment carryover is frequently deleterious to process operation, re-entrainment may be considered as the limiting operating condition for a demisting or mass transfer filamentary pad system. Typically, for a given pad, the re-entrainment point occurs at lower gas and liquid loadings than does the flood point.
Conventional knitted-mesh filamentary pads used for mist elimination are comprised of mesh layers with a uniform knitted pattern with limited variation in mesh opening sizes. In the typical multi-layered mesh pad used in mist eliminator or mass transfer applications, the medium that the gas “sees” is therefore a substantially homogeneous three-dimensional network with little or no variation in fluid flow resistance. This is also true of woven-mesh mist eliminators pads, such as Pedersen, U.S. Pat. No. 4,022,596.
At low liquid loadings and gas velocities in conventional filamentary pads, used for mist elimination or mass transfer, separate flow channels for the gas and liquid establish themselves. At moderate and high gas velocities and/or mist loads, the flow capacities of the respective channels utilized at low fluid flows are exceeded. The upwardly flowing gas is then forced to rise through some of the same mesh areas that the liquid is using in a downward flow. This competitive counter flow situation impedes liquid drainage flow and typically results in the formation of a liquid layer at the bottom of the pad through which the gas bubbles.
Related references for increasing liquid and gas flow capacity of filamentary pads has focused on methods and apparatus for removing the bottom liquid layer in the filamentary pad by augmenting liquid drainage. Typical of this art is Lerner, U.S. Pat. No. 4,022,593, and Ozolins, et al., U.S. Pat. No. 4,744,806. Lerner, '593 and Ozolins, et al., '806, both provide preferential discharge paths to drain the liquid from the filamentary pad to avoid liquid flow interference with gas flow paths in the pad. Lerner provides external filamentary drainage rolls that act as appended liquid downspouts. Additionally, the drainage rolls employ the Coanda effect to inhibit formation of the bottom liquid layer in the pad.
Ozolins, et al. uses mesh sections of different densities so placed that they form defined substantially vertical zones of varying density, i.e., controlled density zones in the direction perpendicular to gas flow. Higher pad densities correspond to higher liquid capillarity and higher gas flow resistance; so that the vertical high-density pad sections define preferred liquid drainage paths. The structured mesh pads of Ozolins, et al., cannot be made by the conventional stacked horizontal layer assembly method. They are best made by spiral or annular assembly construction methods, which are complicated and expensive to build and are therefore limited to smaller pads sizes. The drainage roll appendages of Lerner also require additional fabrication steps beyond a conventional layered pad assembly that add to the cost and complexity of construction.
The present invention overcomes the disadvantages of the related art as described below.